In 1968 Thomas, Chess, and Birch focused their studies in psychology on children, and their specific behaviors. Their hypothesis was that the personality of children is classifiable, and that it can also be predicted judging from the choice of behavior of each child.

Hence, the study of "temperament" or the unique set of personality traits that separate one child from another but that also collectively organize them by the type of trait.

The conclusion of their study was that there are nine behaviors in children that are almost ever-present.

Adaptability- kids in general tend to adapt to the "playground rules" even when this involves interacting with children that speak a different language, for example, putting up with the bullies, or following an alpha.

Regularity or Rhythmicity- kids are better when put in a routine. If not in a routine, they develop their own.

Activity Level - children without increased activity levels are not typical

Attention span- all children show growths in attention span depending on activity and focus

Distractability - children distract because their attention span is short.

Intense reaction- children exaggerate their pains and over-react at many things because of their fear of unknown situations.

Quality of mood- children have the capacity of acting as if they were bipolar: crying one minute, laughing the next minute...but there is real reason behind these moods. They are just so intense that they consume themselves fast enough to be able to switch to another mood.

Approach or Withdrawal- some kids are more trusting of adults and strangers than others.

Basically, in either one child or another these behaviors will surface at some point depending on what the child is doing. This being said, they also found out that those 9 indicators result in three types of children:

easy

difficult

slow-to-warm-up

Keep in mind that they are speaking of a "majority" of up to 60% of children falling into one of these three categories, but there are still traits that may set some children still farther apart. For example, the current rise in autism may render the classic temperamental categories prone to become revisited, as children with autism and Asperger clearly do not fall within any of the three categories proposed by Thomas, Chess, et al.